Offset lithographic printing has remained a most popular method of printing for many years. An important reason for this is the relative ease with which offset lithographic printing plates can be produced. Currently, the most widely used method for plate preparation has remained that which utilizes specially prepared masking films through which pre-sensitized printing blanks are selectively hardened or softened (according to the chemistry of the plate) by exposure to ultra violet light. The plate then undergoes a development process, during which the more soluble regions of the plate are washed away. A detailed description of the system and the plates used can be found in Chapter 20 of the book Printing Materials: Science and Technology by Bob Thomson 1998, published by Pira.
In recent years, various considerations have arisen that point to advantages for modification of hitherto generally accepted practices. With the advent of computers, information for printing is prepared digitally and it has become preferable to use this digital information as directly as possible in plate preparation. One obvious way would be to eliminate the masking film. Not only are these films a source of expense, but the most widely used films are based on silver chemistry whereby the exposure and handling of the film must be in a light-excluding environment. In addition, the exposed film must be processed with chemical solutions, which are unstable, messy and environmentally problematic.
One method which avoids these problems is found in computer-to-plate (CTP) systems where the offset litho plates are directly imaged with a light source that is modulated to correspond to the digital information from the computer. Thus the film intermediate is completely eliminated. In general, such plates still need processing by solution although attempts are being made to develop computer-to-plate systems that are processless. The subject of computer-to-plate can be found in the above quoted book in Chapter 21.
U.S. Pat. No. 5,339,737 to Lewis et al describes the processless preparation of offset litho printing plates, wherein the upper layer or layers of the plate are ablated away. The upper layer is either oleophobic for waterless plates or hydrophilic for conventional wet process plates. The substrate is oleophilic in both cases. U.S. Pat. No. 5,353,705 to Lewis et al is similar to the previous patent but describes additional layers for secondary partial ablation. U.S. Pat. No. 5,487,338 is similar but includes reflective layers. All of these inventions involve multiple layered plates that are expensive to produce. Also, it is difficult to maintain a consistent standard of quality from plate to plate. Moreover, they utilize laser imaging systems which are in themselves costly.
Therefore, it would be desirable to have a simplified, quick plate making process with elimination of all chemical processing and a minimal cost for the equipment for plate production and for the plate production itself.
A technology that has been developed in recent years may be seen to offer solutions to easy and inexpensive plate production. This is the technology of inkjet printing.
Inkjet is a non-impact printing process whereby ink is sprayed through very fine nozzles and the resultant ink droplets form an image directly on a substrate. There are two main types of ink jet processes. In one process, usually termed continuous ink-jet printing, a stream of ink drops are electrically charged and then are deflected by an electric field either directly or indirectly onto the substrate. The viscosity of inks used in such systems is typically 2 or 3 centipoise. In the second process, usually called Drop on Demand (DOD) inkjet printing, the ink supply is regulated by an actuator such as a piezoelectric actuator. The pressure produced during the actuation forces a droplet through a nozzle onto the substrate. Inks for DOD ink-jet printing do not need to be conductive and their viscosity is typically between 2 and 40 centipoise.
Several inventors have tried to apply the principles of ink-jet printing to offset plate making. U.S. Pat. No. 4,003,312 was one of the first patents to recognize the advantages of using inkjet printing technology in a process for preparing a waterless lithographic printing plate. This patent discloses the use of an inkjet printing apparatus to deposit a background coating of silicone being curable.
U.S. Pat. No. 4,718,340 discloses a method for preparing a reusable planographic plate for lithography printing wherein a hydrophilic substrate is provided with a thin hydrophobic layer which is selectively removed. This process involves a multi-step plate preparation using hydrophobic organic acids and derivatives thereof. The treated surface is then selectively imaged with a spark discharge, or laser ablation technique. This has limited run length capability, similar to other spark discharge and laser ablation techniques.
U.S. Pat. No. 5,312,654 discloses a method for making a printing plate wherein an ink absorbing layer is selectively imaged with photopolymerizable composition using an inkjet printer. The ink absorbing layer prevents the ink from spreading and is removed after the ink is cured by exposure to actinic light, thereby exposing a hydrophilic surface where photopolymer has not been deposited. This process is impractical because the water-soluble or alkali-soluble coating used in the ink-absorbing layer has serious disadvantages since the ink imaged photopolymer sits on top of this layer. On typical offset press, the use of an aqueous fountain solution would be disastrous for this plate. Additionally, the ink absorbing properties of this film limit control of dot or image formation and the resolution of fine details is still problematic.
U.S. Pat. No. 5,750,314 to Fromson et al. describes selective positive imaging on a substrate coated with a first continuous layer of a material, and further coated by inkjet with a second material, which is adhesive to the first layer. The difference in solubility of each material in a specific solvent enables developing and removing the non-imaged layer of the first material. In this method the substrate is made hydrophilic. The first material is preferably a negative working photopolymer. The disadvantage of this method is that after masking the imaged area, the plate needs to be exposed and developed.
Japanese Patents JP10,157,053, JP 10,076,624 and JP 10,024,549 describe a method which involves injecting oily ink through a nozzle and forming an image on an image forming layer of an original plate. Resin is impregnated in the non-image formation area of the image-forming layer. The non-image formation area is desensitized by subjecting the resin to chemical reaction, to form a flat printing plate.
JP 10,076,625 involves performing thermofusion of solid ink composition at normal temperature. Ink is injected from a nozzle on to an intermediate transfer object and an image is formed. Then, contact transfer of the image from the intermediate transfer object to the image acceptance layer of an original plate is carried out. The area where the image is not formed in the image acceptance layer is processed by chemical reaction and it is formed on the waterproof support body that has resin.
U.S. Pat. No. 4,833,486 to Zerillo (assigned to Dataproducts) utilizes a hydrophobic solid inkjet ink (containing waxes) which is held at a sufficiently high temperature to jet it through a DOD head. (This solid ink technology is more fully described in U.S. Pat. Nos. 4,390,369, 4,484,948, and 4,593,292.) The substrate is a hydrophilic offset plate—either paper or aluminum onto which the image is jetted. When the ink hits the plate it immediately cools and solidifies. One problem of such an approach is the difficulty of obtaining sufficiently good adhesion of the waxes of the ink to the plate to run multiple impressions during lithographic printing.
European Patent EP503621 (Applicant NIPPON PAINT CO) describes two approaches. One approach describes jetting inks onto a pre-sensitized plate, which then needs further treatment, including a developing stage with a liquid developer. The other approach uses a non-presensitized plate and the inkjet ink is photosensitive so that it can be hardened on the plate.
EP533168 to Nippon describes the use of a photopolymeric based inkjet ink together with an ink absorbing layer on the litho plate surface.
EP697282 to Leanders (Agfa) describes a two component system whereby one reactive component is in the ink and the other in the litho plate surface, so that when the ink is deposited on the plate it produces an oleophilic reduced silver image that can be used in the offset printing process.
U.S. Pat. No. 5,495,803 to Gerber describes imaging a coated, presensitised plate with a UV opaque hot-melt inkjet ink and using the ink as a photomask to expose the plate. The unexposed presensitised polymer and the ink are subsequently removed by washing.
U.S. Pat. No. 5,738,013 to Kellet describes an ink-jet plate-making process involving the use of a reactive inkjet ink which is bonded to the litho plate by a chemical reaction activated by radiant energy. This assumes that such inks have very good stability at room temperature so that no jet blocking will occur, yet have good reactivity at high temperatures so that the ink becomes insoluble with good adhesion to the offset plate and with good oleophilic properties.
Another option is coating the substrate with solution containing cationic surfactants, as described in U.S. patent application No. 60/174,713, assigned to the owners of the present invention. According to this method the surface of a substrate, bare anodized aluminum with no pre-coating (as polymeric binder that should be washed away), is coated with a very thin layer (almost mono-molecular) of cationic surfactants. The coating is water repellent and insoluble in the CTP liquid. The plate is then imaged using an inkjet printing head providing an excellent image quality and a strong stable oleophilic image from which to print high numbers of good quality impressions.
However, these prior arts attempts to use the inkjet process for imaging plates, remain with difficulties in producing satisfactory quality, run length and plate-making speed, because of problems of spreading and clustering.
In order to produce high-resolution plates at high speed it is necessary to position large number of droplets in rapid succession very close together. Creating an image on a highly hydrophilic, water receptive surface of an anodized aluminum offset plate with water-based liquid by means of an ink jet process, is very problematic. The high surface energy of the anodized grained aluminum causes an intensified spreading of the liquid drops. Therefore, it is almost impossible to create a sharp image on the plate. Both water-based and solvent-based inks have problems of spreading of the liquid on the high surface energy hydrophilic plate surface due to the properties needed to jet the ink.
In addition, water-based inks, due to the hydrophobic nature of the plate's coating, tend to create a clustering phenomenon at the dark tone areas of the image (shadows). Prior art methods, in order to contend with the problem of clustering, have required multiple passes of the inkjet head with a drying step between the passes. This makes the plate preparation time quite long.
One option of controlling the spreading of drops is by controlling the viscosity of the CTP liquid, as described in Israel patent application No. 132789 and the parallel PCT application PCT/IL 00/00722. This application describes the use of polymer emulsion in water to produce good quality long run lithographic printing plates. Yet the resolution received is plate dependent, i.e. with certain substrates this is more successful than with others.
U.S. Pat. No. 4,381,85 discloses a simple process to obtain a water-fast print on paper using a colorant solution containing water-soluble polymeric dyes. This is accomplished by using a paper employing substantial cation content, especially a substantial polyvalent metal ion content for example, Fe2+, Fe3+, Cu2+, Zn2+, Al3+, Mg2+, Ca2+and Ba2+, and applying a solution containing one or more polymeric colorants possessing anionic net charge, for example, anionic groups. The reaction between the cations and anions immobilize the ink drops on the surface where they are applied so that spreading does not occur.
It would be desirable to provide an offset printing plate for direct inkjet CTP which would not have the problems of spreading and clustering and which would provide an easy, economical method for plate preparation.